Final: Ch 17 Myosin Flashcards
what are myosins
actin-based motor proteins that move along actin filaments
convert energy from ATP hydrolysis into movement along actin
myosin II
found in skeletal muscle
hundreds of molecules assemble into bundles
myosin V
binds vesicular cargo to transport it along actin filaments
domains of myosins
head
neck
tail
how to dissolve a myosin thick filament
solution of ATP and high salt
myosin II domains
2 heads and long tail connected by a flexible neck
tails intertwine and heads close together
myosin II has what type of activity
ATP hydrolysis (ATPase) in the S1 fragment
S1 also has F-actin binding site
all myosins have ___-_______ ATPase activity
actin-activated ATPase activity
sliding-filament assay
myosin attached to glass slide
actin filaments stained with phalloidin and ATP added
myosin heads walk toward + end, so looks like - end movement leads
tail domain
tailored to bind specific cargo
which domains of myosins are responsible for motor activity
head and neck (S1)
all myosin head domains convert what
ATP hydrolysis into mechanical work
myosin II assembles into _____ filaments
bipolar filaments
opposite orientations in each half so theres a cluster of head domains at each end
does myosin II have a short or long neck
short
myosin I
single headed
many functions
ex. connect actin filaments to membranes, endocytosis
myosin V domains
two heads and a long neck
tails dimerize
the length of the neck region affects the rate of myosin __________
movement
myosins move toward which end of the actin filament, except 1 myosin which moves toward the other end
+ end
myosin VI moves toward the - end
membrane-associated actin has their + end toward what?
the membrane
function of multinucleated skeletal muscle
produce gene products all along the cell
myosin heads do what along actin filaments
slide/”walk”
the hydrolysis of a single ATP molecule is coupled to each _____ taken by a myosin molecule along an actin filament
step
process of ATP-driven myosin movement along actin filaments
without ATP, the head is attached to F-actin
ATP binds the head, which releases the filament and hydrolyzes ATP –> ADP + Pi causing rocking
the myosin head binds actin
release of Pi is coupled to moving the actin filament forward (left) “power stroke”
ADP is released
the neck acts as a _____ point
pivot point
the myosin step size is proportional to what
neck length
tested by manipulating the genes for neck length (altered step size) and optical trap
step length of myosin II compared to myosin V
myosin V step length is 10x longer b/c of a very long neck (like a tether)
optical trap of myosin II
myosin on beads interacts with an actin filament held between 2 optical traps
when ATP is added, the myosin pulls the actin and a computer records the distance
what did the optical trap of myosin II show
that myosin II binds, moves, and releases the actin
it doesn’t bind continuously
processive movement
myosin V takes sequential steps without releasing from actin
one head is always in contact with the actin
does myosin V walk hand over hand on an actin filament?
yes
can myosin V transport organelles?
yes
do microtubules have classes that move in opposite directions?
yes
what do the tail regions of myosins determine
what kind of cargo it carries
myosin ____ filaments and actin _____ filaments in skeletal muscle slide past one another during _______
myosin thick, actin thin, contraction
skeletal muscle fiber properties
long and multinucleated
striated
sarcomeres
smooth muscle fiber properties
no sarcomeres
mononucleate
non-striated
sarcomere
arrangement of actin/myosin
shortens by 70% during contraction
thick filaments are composed of what
myosin II bipolar filaments
move toward Z disk where + ends are, causing contraction of the muscle
skeletal muscle is structured by ________ and ________ proteins
stabilizing, scaffolding
actin capped by CapZ on the + end, and tropomodulin on the - end
nebulin goes from Z disk to tropomodulin (where it binds)
tintin is elastic and prevents overstretching
skeletal muscle contraction is initiated by an increase in cytosolic __
Ca2+
in skeletal muscle, a low cytosolic Ca2+ level is maintained by…
an ATPase that pumps Ca from the cytosol and into the sarcoplasmic reticulum
an action potential opens what in skeletal muscle
voltage-gated Ca channels in the SR membrane to raise cytosolic Ca concentration and induce muscle contraction
what effect does Ca have on skeletal muscle (thin-filament regulation)
changes tropomyosin and troponin which normally block myosin binding
allows myosin-actin interactions and therefore contraction
contractile bundle
bundles composed of actin and myosin that are found in nonmuscle cells
less organized than skeletal muscle fibers
in epithelial cells, contractile bundles are found as an ______ _____
adherens belt
3 types of contractile bundle
adherens belt - epithelium
stress fibers -ECM
contractile ring - cell division
skeletal muscle contraction is regulated by the ________-_______ complex bound to thin actin filaments
tropomyosin-troponin complex
switches between contraction inducing state in the presence of Ca, and the relaxed state without Ca
how is smooth muscle contraction regulated
cycling of myosin II in response to extracellular signaling molecules
phosphorylation/dephosphorylation of myosin regulatory light chain by myosin LC kinase (thick-filament regulation)
myosin V bound vesicles are carried along _____ filaments
actin
myosin V in budding yeast
carries secretory vesicles along actin filaments into the bud
distributes organelles
myosin V in giant green algae
cytoplasmic streaming
myosin V carries ER, which also moves cytoplasm
cell migration results from what
coordinated motions generated in different parts of the cell integrated with an endocytic cycle
how is cell migration initiated
formation of a large membrane protrustion at the leading edge of the cell - lamellipodium
cell migration of a fibroblast
membrane extension occurs at the leading edge where the network of actin filaments is nucleated by Arp2/3 (forms lamellipodium)
some lamellipodia anchor to the substratum via focal adhesions
the cell body is translocated forward by myosin II cortical contraction at rear of cell
focal adhesions at the back of the cell are broken and integrins recycled by endocytosis
freed tail snaps forward
the cell-adhesion molecules that mediate most cell-matrix interactions are membrane proteins called ______
integrins
external domain binds ECM (fibronectin and collagen)
cytosolic domain links them to the actin cytoskeleton
2 importances of focal adhesion attachment
prevents leading lamella from retracting
attaches cell to the substratum, allowing for forward movement
what 3 small GTP-binding proteins control actin organization
Rho
Rac
Cdc42
what do growth factors like epidermal growth factor and platelet-derived growth factor do
bind cell surface receptors and stimulate cells to move and then divide
ex. in a wound, platelets become active and secrete PDGF to attract fibroplasts and epithelial cells to the wound
what effect does dominant negative Rac have on wound healing
wound fails to close b/c Rac is needed to activate the Arp2/3 complex to form a lamellipodium
Cdc42 is critical for regulating ________ of cells
polarity
dominant negative results in formation of leading edge, but movement in random directions
migrating cells are steered by _______ molecules
chemotactic
ex. WBC are guided toward an infection by a tripeptide secreted by bacterial cells
common mechanism of chemotaxis
binding cell surface receptors
activate intracellular signaling pathways
remodel cytoskeleton through activation or inhibition of actin binding proteins
chemotaxis and amoebas
when stressed, these amoebas secrete cAMP which causes them to move toward each other
they them form a slug and make starvation-resistant spores
membrane recycling in cell migration
membrane needed to form lamellipodia is provided from internalization of adhesion molecules (including integrins) from old focal attachments to front of cell
dominant active protein
mutant small GTPase locked in the active state
dominant active Cdc42
appearance of filopodia through Arp2/3 activation via the NPF WASp
dominant active Rac
membrane ruffles/lamellipodium
dominant-active Rho
stress fibers that contract
general model of how cell migration is controlled via Rac, Rho, Cdc42
Cdc42 orients cell
oriented cell has high Rac activity in front to form lamellipodium
Rho activity is high in the rear to assemble contractile structures and activate myosin II
